A light-emitting element is a so-called organic electroluminescent element in which, by an electric current flowing between the cathode and anode, an organic compound between both electrodes is made to emit light.
A general sectional structure of a light-emitting element is shown in FIG. 1. In the FIG. 1, 1 denotes a transparent substrate, 2 a transparent electrode (anode), 3 a hole transporting layer, 4 a light-emitting layer, 5 an electron transporting layer, 6 an electron injecting layer, and 7 a cathode.
In this light-emitting element, an exciton is generated by the recombination of the electron, injected to the light-emitting layer 4 from the cathode 7 through the electron injecting and transporting layers 5 and 6, and the hole injected to the light-emitting layer 4 from the transparent electrode 2 through the hole transporting layer 3. The light-emitting is an element which takes advantage of the light emitted when the exciton returns to the ground state.
For the cathode 7 of such a light-emitting element, there is used a material which has a relatively small work function and satisfactory electron injection characteristics, for example, an elemental metal such as magnesium (Mg) or a metal alloy such as Ag—Mg and Al—Li alloys.
In addition, patent document 1 discloses a configuration in which an organic layer containing a metal functioning as a donor (electron donating) dopant is provided in contact with the cathode. As the metal used as the donor (electron donating) dopant, patent document 1 discloses alkali metals, alkaline earth metals, and transition metals inclusive of rare earths.
In addition, patent document 2 discloses a configuration in which an organic layer having a metal oxide or a metal salt as a dopant is provided in contact with the cathode.
Patent document 1: Japanese Patent Application Laid-Open No. 10-270171 (page 2, lines 9 to 13, and FIG. 1).
Patent document 2: Japanese Patent Application Laid-Open No. 10-270172 (page 2, lines 2 to 7, and FIG. 1).
As these dopants, metals each having a work function as small as possible, and the oxides and metal salts containing such metals are preferable. Such a metal is generally high in reactivity, and hence handling such a metal is very tough. In addition, when a metal oxide or a metal salt containing such a metal is used as a dopant, the dopant becomes higher in stability, but still some of the metal oxides and metal salts are unstable so that handling thereof in the usual air environment is not easy. On the other hand, some others of the metal oxides and metals salts can be handled in the air, but they are still unstable so that it is difficult to introduce them each as a dopant into the organic layer provided in contact with the cathode, as the case may be. Accordingly, the uneasy handling of them as the dopants can be a cause for the low process yield and the cost rise in the production of the light-emitting element.